The invention relates to methods for forming crystalline gallium nitride. In particular, the invention relates to methods for high temperature growth of crystalline gallium nitride in a supercritical solvent.
Crystalline gallium nitride is useful as a material for applications in electronic devices including, but not limited to, light-emitting diodes and laser diodes. Currently, gallium nitride (GaN) crystal size and growth, which are produced by known processes, are adequate for some applications, however for many other applications, the gallium nitride crystalline size and quality are not adequate.
Several processes are currently used to produce gallium nitride crystalline substrates. The processes include heteroepitaxial growth of gallium nitride on a substrate, such as a sapphire or silicon carbide. The heteroepitaxial growth process often results in defects, which include, but are not limited to, high concentrations of at least one of dislocations, vacancies, and impurities. These defects may have undesirable and detrimental effects on epitaxially grown gallium nitride, and may adversely influence operation of the resultant gallium nitride-based electronic device. These adverse influences include compromised electronic performance and operation. Presently, heteroepitaxial gallium nitride growth processes require complex and tedious steps to reduce defect concentrations in the gallium nitride.
Known gallium nitride growth processes do not provide large gallium nitride crystals, for example gallium nitride crystals greater than about 0.8 inches (about 2 centimeters) in diameter or greater than about 0.01 inches (about 250 microns) in thickness. Further, the known methods are not known to provide for production of large gallium nitride crystals that result in single-crystal gallium nitride boules, for example gallium nitride crystals of about 1 inch in diameter and about 0.5 inches in thickness, which are suitable for forming wafers. Thus, applications for gallium nitride are limited due to size constraints.
Also, most known gallium nitride crystal production processes do not provide high-quality gallium nitride crystals that possess low concentrations of impurities and dislocations with adequate size and growth rates for electronic device applications. Further, the known gallium nitride crystal production processes are not believed to provide an economical process with nitride growth rates that enable moderate-cost gallium nitride crystal production. Therefore, applications for gallium nitride are further limited due to quality and cost-of-production factors.
Small gallium nitride crystals, such as platelets and needles, have been grown by reaction of nitrogen (N2) gas with gallium (Ga) metal at pressures in a range from about 10 to about 20 kbar and at temperatures in a range of about 1200xc2x0 C. to about 1500xc2x0 C. The gallium nitride crystalline quality produced by this process may be adequate, in terms of dislocation density, for some gallium nitride applications. The gallium nitride crystalline quality formed by this process, however, exhibits a high concentration of undesirable nitrogen-vacancy defects, which adversely influences certain gallium nitride crystal applications. Additionally, this process appears to be limited to producing a maximum gallium nitride crystal size of about 15 millimeters (mm) to about 20 mm in diameter and only about 0.2 mm in thickness. This gallium nitride production process may also suffer from small gallium nitride crystal growth rates, for example growth rates of about 0.1 mm/hr.
Small gallium nitride crystals, for example in the form of crystalline platelets and/or needles with a size less than about 0.4 millimeters (mm), have been grown in supercritical ammonia (NH3) in pressure vessels. These supercritical ammonia growth processes exhibit slow growth rates, and thus do not enable boules or large gallium nitride crystals to be readily produced. Also, the pressure vessels limit these gallium nitride growth processes. The pressure vessels limit the supercritical ammonia growth process to a pressure less than about 5 kbar, and thus limit the supercritical ammonia growth process temperature and reaction rate.
Gallium nitride growth on an existing substrate has been proposed by a chemical vapor deposition (CVD) process. The CVD process may use reactions, such as, but not limited to, GaCl+NH3 or Ga(CH3)3+NH3. These CVD processes are believed to be limited by at least one of: limited capability for growing large, thick gallium nitride crystals and substrates; poor gallium nitride crystal quality due in part to the use of an existing substrate, such as sapphire and silicon carbide, that may result in an undesirable lattice mismatch; and subsequent low gallium nitride crystal growth rates. These CVD process limits may lead to high costs of gallium nitride growth, which, of course, is undesirable.
Further, gallium nitride growth from other processes, such as reacting of gallium and NaN3 at elevated pressures, atmospheric-pressure flux growth, and metathesis reactions (GaI3+Li3N) have been proposed. These proposed growth processes are believed to be costly, and are not believed to produce high-quality, defect free gallium nitride in crystalline form.
Therefore, a gallium nitride crystal growth process that produces gallium nitride crystals of high quality is needed. Further, a gallium nitride crystal growth process that can produce large gallium nitride crystals is needed.
A gallium nitride growth process forms crystalline gallium nitride. The process comprises the steps of providing a source gallium nitride; providing mineralizer; providing solvent; providing a capsule; disposing the source gallium nitride, mineralizer and solvent in the capsule; sealing the capsule; disposing the capsule in a pressure cell; and subjecting the pressure cell to high pressure and high temperature (HPHT) conditions for a length of time sufficient to dissolve the source gallium nitride and precipitate it into at least one gallium nitride crystal.
Another gallium nitride growth process for forming crystalline gallium comprises providing of providing a source gallium nitride; providing mineralizer; providing solvent; providing a capsule; disposing the source gallium nitride, mineralizer and solvent in the capsule; sealing the capsule; disposing the capsule in a pressure cell; and subjecting the pressure cell to high pressure and high temperature (HPHT) conditions for a length of time sufficient to dissolve the source gallium nitride and precipitate it into at least one gallium nitride crystal; cooling the high pressure and high temperature (HPHT) system; relieving the pressure in the high pressure and high temperature (HPHT); removing the gallium nitride crystals from the high pressure and high temperature (HPHT) system; and washing the gallium nitride crystals in at least one of water and mineral acids.
A further gallium nitride growth process for forming crystalline gallium nitride comprises providing a capsule that comprises two opposed end units; disposing a seed gallium nitride crystal in one end unit of the capsule; disposing source gallium nitride with mineralizer and solvent in the other end unit of a capsule; disposing solvent in each of the capsule end units; sealing the capsule; disposing the capsule in a pressure cell; and subjecting the pressure cell to high pressure and high temperature (HPHT) conditions in a high pressure and high temperature (HPHT) system for a length of time sufficient to dissolve the source gallium nitride and precipitate it into at least one gallium nitride crystal.
A still further gallium nitride growth process for forming crystalline gallium nitride comprises providing solid or liquefied gallium as the source gallium material; providing a capsule that comprises two opposed end units; disposing a seed gallium nitride crystal in one end unit of the capsule; disposing the source gallium with mineralizer and solvent in the other end unit of the capsule; disposing solvent in each of the capsule end units; sealing the capsule; disposing the capsule in a pressure cell; subjecting the pressure cell to high pressure and high temperature (HPHT) conditions for a length of time sufficient to react the source gallium with the nitrogen-containing solvent under the HPHT growth conditions to form gallium nitride; and subjecting the capsule to high pressure and high temperature (HPHT) conditions for a length of time sufficient to dissolve the formed gallium nitride and precipitate it into at least one gallium nitride crystal.
The invention also provides for gallium nitride crystals formed by each of above the above-described processes.
These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, where like parts are designated by like reference characters throughout the drawings, disclose embodiments of the invention.